The present invention relates generally to mixing and dispensing apparatus used in the polyurethane foam industry, and move particularly to a novel means for metering on ratio the flow of foam components through the dispenser.
The use of urethane foams is commonplace in the building industry. Urethane foams are used as adhesives, as insulation material, as roofing components, etc. Most urethane foams used in these applications are the reaction product of two individual components, one being an “A” or isocyanate component and the other being a “B”, or resin component. These components are mixed together to typically form an expansive foam.
In operation, separate containers of the A and B components are used in conjunction with a dispensing gun in which the flow of each of the A and B components is controlled as they pass through the dispensing gun into a mixing and discharge nozzle. U.S. Pat. No. 5,529,245, issued Jun. 25, 1996 and U.S. Pat. No. 5,944,259, issued Aug. 31, 1999 to the assignee of the present invention describe two constructions of foam-component dispensing guns.
In each such construction, the gun has a body portion that at a rear end portion thereof, receives and which houses two inlets connectable to the foam component supply containers. The gun body also contains a mixing and discharge nozzle at a front end thereof. A rotatable spool valve is rotatably mounted in the gun body and has two passages formed in a body portion which provide two pathways between the inlets and the nozzle for mixing and discharge. This valve is rotated when the trigger of the gun is moved toward the gun handle.
This rotation brings the two valve passages into alignment with passages formed in both the mixing and discharge nozzle and the inlets. All of the passages in these known dispensing guns are circular in cross-section. Metering of the foam components discharged through the nozzle is achieved by slowly depressing the trigger and moving it toward the handle. Although this action produces adequate results, it is still difficult to obtain low-flow dispensing of the foam components that might be obtained using a needle valve.
In order to provide foam dispensers that accurately meter and dispense foam at low flow conditions, the industry has utilized needle valves for controlling the flow. In a typical needle valve, a needle is held within a bore and in moved in and out of the passage by means of a moveable stem mounting in order to adjust the size of the annular passage that is formed between the needle and the bore.
Needle valves work very well for controlling the flow of a single stream of a liquid or a gas. It is difficult, however, to couple together the adjustment of two needle valves on two streams of liquid/gas so that an exact ratio between the two streams is obtained. In order to do so, each stream must be activated simultaneously with a consistent rate of change and at similar degrees over the opening and closing of the valves. The viscous and highly reactive nature of the foam components, particularly the isocyanate component, causes the needle to often stick in its bore, necessitating frequent disassembly and cleaning and/or replacement of the needles or even replacement of the entire dispenser. It is also difficult to tie the two needles of the needle valve together in a manner so that movement of one needle effects a similar movement of the other needle to obtain the needed ratio.
Also, the phenomena of “lead lag” has been noted in the use of foam dispensers that use needle valves. These occur at low-flow conditions when one of the two needles unseat ahead or after the other when this occurs, it creates an “off-ratio” situation where the amount of one flow component does not match the desired amount that must combine with another desired amount of the other flow component. As such, the desired ratio of the two reactive foam components is not met, thereby resulting off-ratio foam in which the two reactive components do not completely combine. This lead lag is also affected by the viscosities of the foam components, and the rate and velocity of the flow. This is caused by the aforementioned inability to effectively tie the two needles together so that precise seating and unseating of the needles will occur.
The present invention is therefore directed to a dispensing apparatus with an improved flow metering assembly that overcomes the aforementioned disadvantages.